The broad, long- term objective of the proposed research is to investigate and understand the principles of perceptual organization. Ever since the seminal work of Gestalt psychologists in the 1920s, the topic of perceptual organization -- essentially, the nature of part-whole relations among perceptual elements -- has been recognized as central in perceptual theory, but it has been curiously resistant to integration with modern theory. The investigator is reexamining some of the basic assumptions of the Gestalt work and finds them wanting. Specifically, the well-known "laws of grouping" are widely believed to operate early in vision and to define perceptual units, but the investigator currently believes that both of these propositions are false. Instead, he is finding that grouping is determined by relatively late processes that operate at or after perceptual constancy. He also argues that logic dictates a very different organizational principle -- which he calls uniform connectedness -- to create the perceptual units on which grouping processes operate. In the investigator's view, uniform connectedness provides initial access to the part-whole hierarchy of perceptual organization at what he calls the entry level. He proposes to continue his preliminary studies testing empirical predictions based on these and related theoretical ideas. He plans to use both standard subjective report methods and several novel reaction-time techniques that provide quantitative measures of performance on objectively defined tasks, including the repetition discrimination task and the primed matching task.